Proximity indication with range and bearing measurements

ABSTRACT

Proximity indication and evaluation for aircraft, using only the signals emitted by secondary surveillance radar and cooperating transponders, to detect intrusion in a monitored proximity volume and determine slant range and relative bearing to the intruder.

This application is a division of application Ser. No. 180,578, filedSept. 15, 1971 now U.S. Pat. No. 3,757,324.

BACKGROUND

1. Field of the Invention

This invention pertains to radiolocation of mobile vehicles, such asaircraft, with respect to each other within the coverage of a scanningradar at a reference location.

2. Description of the Prior Art

Major airports and way points are presently equipped with secondarysurveillance radar (SSR) adapted to cooperate with transponder beaconscarried on aircraft to discriminate against interference and groundclutter and to provide for transmission of identification and other datasuch as altitude from the craft to the groundbased radar. A trafficcontroller observing the radar display directs the pilots of theinvolved aircraft by radio, usually with voice communication, so as tomaintain or restore safe separations between craft. Such systems arelimited in capability because each craft must be dealt with individuallyand requires its share of the controller's time and attention and itsshare of the available radio spectrum. When traffic is heavy, takeoffsand landings are delayed, and the possibility of collisions increases.

The number of mid-air collisions and near misses has become so large inbusy areas that numerous interaircraft cooperative proximity warningsystems have been proposed. Those more prominently under study ordevelopment at this time involve frequent or quasicontinuous exchange ofsignals between all cooperative aircraft within the region of interestand make no provision for non-cooperating aircraft. The requiredairborne equipment would be bulky and expensive, use more of the alreadycrowded radio spectrum and would be generally independent of otherneeded and existing equipment, such as transponders. Another drawback ofsome of the proposed systems is that they provide only relativepositional information, without ground reference but in effect withrespect to a randomly floating reference.

My copending U.S. patent application Ser. No. 130,952, filed Apr. 5,1971, now U.S. Pat. No. 3,735,408 and entitled Common Azimuth SectorIndicating System describes the use of a standard airborne transponderwith additional equipment including a receiver for receiving other'stransponder replies and means for indicating the presence of another.[.transponderequipped.]. .Iadd.transponder-equipped .Iaddend.aircraftwithin a monitored airspace sector or volume.

SUMMARY

According to the present invention, detection of a proximity situationas in the above-identified U.S. Pat. No. 3,735,408 is used to add aspecial proximity code signal to the normal transponder reply. Such asignal, when received from another similarly equipped aircraft,initiates an exchange of interrogations and replies between therespective transponders of a proximity pair, enabling measurement of thedirect slant range between the two craft. This operation may beaccomplished without modification of the standard SSR-transpondertraffic control system, and without interference with its normaloperation.

The slant range information, useful in itself as a quantitative measureof the degree of proximity, may also be used in the determination ofrelative bearings of the two aircraft from each other. To this end, theSSR must be arranged to transmit .[.onmidirectionally.]..Iadd.omnidirectionally .Iaddend.a reference signal, called a "Northpulse" when the sweeping main radar beam points toward the localmagnetic North. North pulse reference signals are available at existingSSR installations, and can be transmitted as required with slightmodification of the equipment.

The North pulse, received and decoded by the transponder when the beampoints North, and the normal interrogation, received when the beampoints at the aircraft, define a time interval which is a measure of the.[.carft's.]. .Iadd.craft's .Iaddend.own magnetic bearing from theradar. The own radar bearing, slant range, and the differential rangefrom the radar, which can be determined from the interval between aradar interrogation and the reception of the other's reply to the sameinterrogation, provide sufficient data for a simple computation ofother's relative bearing.

DRAWINGS

FIG. 1 is a block diagram illustrating generally a preferred embodimentof the invention.

FIG. 2 is a geometrical diagram used in explaining the operation of theapparatus of FIG. 1.

FIG. 3 is a more detailed block diagram showing a specificimplementation of the embodiment of FIG. 1.

FIG. 4 is a block diagram of a PRF selector suitable for use in thesystem of FIG. 3.

FIG. 5 is a block diagram of a phase locked PRF generator used in thesystem of FIG. 3.

FIG. 6 is a block diagram of an interval timer used in the system ofFIG. 3.

FIG. 7 is a block diagram of a lead-lag logic device used in the systemof FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a transponder 60 is arranged to receive the usual1,030 MHz interrogations from secondary surveillance radars, and totransmit 1,090 MHz replies in response thereto. Many transponders areprovided with terminals where the P2 of SLS pulse of the interrogationand the first frame pulse F1 of the reply are available; others may bereadily modified for external access to these pulses. A specially codedNorth reference signal, for example a PO pulse preceding the standard P1interrogation pulse while the SSR main beam points North, is madeavailable by slight modification of the transponder decoder, or byaddition of a simple special decoder.

All transponders include reply encoders which may be set either manuallyor by electrical inputs to add any of 4096 coded messages to the replysignal. Some of the available code groups are used to transmitidentification, barometric altitude, and various emergency or situationmessages; many are not used presently. The transponder 60 is providedwith an input line 61 connected to encode a special range commandmessage on the replies when it is energized, for example, in response tothe detection of the proximity of another aircraft's transponder.Another input line 62 is connected to a point in the transponder wherethe reply trigger pulse usually appears. When a pulse is applied to line62, the transponder is triggered to send a reply in the same manner asif it were interrogated by an SSR, although such interrogation is notreceived.

A 1,090 MHz receiver and decoder 63 is adapted to receive and decodetransponder replies of other aircraft in the general vicinity. Thedecoder portion of this device provides an output pulse on line 64 inresponse to the reception of both reply frame pulses F1 and F2 fromanother aircraft's transponder, and an output pulse on line 65 inresponse to the reception of a range command message.

A widened common azimuth sector proximity detector system 66, which maybe the same as that described in the abovementioned copending U.S. Pat.application Ser. No. 130,952, now U.S. Pat. No. 3,735,408, receives thereply frame decode pulse on line 64 and the own transponder F1 pulse andP2 decode pulse on lines 67 and 68. Output from the proximity detector66 energizes the transponder input line 61 to encode proximity andenergizes a proximity indicator 53.

An alternative F1 input to the proximity detector 66 is provided undercertain conditions by a phase locked PRF pulse generator 69, which canbe synchronized to the pulse repetition frequency of a selected radareither by the repetitive burst of F1 pulses resulting frominterrogations received from that radar, or by the continuous train ofdecoded P2 pulses received from that radar within the SLS area. Theoutput of the PRF generator 69 is a "synthetic F1" pulse, coincidentwith the actual F1 pulse when it is present, and substituting for theactual F1 pulse as a time reference when it is absent.

The actual and synthetic F1 pulses are applied to a range computer 70,which also receives as inputs the reply frame decode pulse on line 64and the proximity decode pulse on line 65. The range computer 70, aswill be described in detail with reference to FIG. 3 later, utilizesmeasurement of the time interval between the F1 and proximity decodepulses, when said pulses are present, to determine the direct slantrange Y between the aircraft of a proximity pair. The range computeralso utilizes measurement of the interval between synthetic F1 pulsesand next following reply frame decode pulses to determine the differenceX between the ranges of the two craft from a selected SSR.

Referring to FIG. 2, the selected SSR is at point 71, the "own".[.).].aircraft is at point 72, and the other aircraft is at point 73. Theslant range Y is proportional to the time required for a radio signal totravel from one aircraft to the other and back again, less systemdelays. The differential radar range X is proportional to the timeinterval between reception of an interrogation from the SSR by the owncraft at 72, and reception of the same interrogation by the other craftat 73.

Returning to FIG. 1, the computed slant range Y is displayed by a meteror other quantitative indicator 74. Representations of X and Y areapplied to an other's bearing computer 75, which utilizes these and theoutput of an own SSR bearing computer 76 to determine the bearing of theother craft from one's own craft.

The own SSR bearing computer 76, as will be described further below,utilizes measurements of the time intervals between successive decodedNorth reference signals, and between North reference signals and nextsubsequent F1 bursts, to determine own magnetic bearing φ from theselected SSR. The other's bearing computer 75, also to be describedlater, determines the angle θ between one's own line of position fromthe radar as θ = cos⁻¹ X/Y, adds own SSR bearing φ, and subtracts ownmagnetic heading H, to determine other's bearing B relative to own'sheading. The angular quantities φ and B are displayed by indicators 77and 78, respectively.

Referring to FIG. 2, it is seen that the arc 79 closely approximates astraight line perpendicular to the differential radar range line X, andalso approximately perpendicular to the own line of position 80.Accordingly, the angle θ is approximately cos⁻ 1 X/Y, within one or twodegrees in a typical situation. As shown in the diagram, B= φ + θ -H.

The system of FIG. 1 operates at all times in the usual manner of anordinary SSR beacon transponder, replying to interrogations receivedduring the dwell time as the main beam of an SSR sweeps by it. Similarreplies from transponders on other aircraft, received and decoded by the1,090 MHz receiver-decoder 63, are ignored unless the other aircraftenters the widened common azimuth sector that is swept by the main beamimmediately before or immediately after the "own" aircraft carrying theequipment of FIG. 1. If these replies, as processed in the detectorsystem 66, define a proximity situation, the transponder 60 adds theproximity message to each of its .[.transmission.]..Iadd.transmissions.Iaddend., thereby alerting the air traffic controlsystem by way of the ground based SSR display.

If the other, or "intruder," aircraft is equipped with areceiver-decoder 63 and a detector system 66, it will also add theproximity message to each of its replies. In the usual case, the twoaircraft will not approach the proximity condition along a common radialfrom the SSR. Accordingly, the rotating radar beam will first illuminateonly one of the aircraft, then possibly both, if they are near enough toa common radial, then only the other. In the unusual case, when theaircraft approach proximity along a common radial from one SSR, theywill nearly always be within operating range of another differentlylocated SSR, and on different radials from that SSR.

When one aircraft is being illuminated by a particular radar beam andthe other is not, the one in the .[.baem.]. .Iadd.beam .Iaddend.will bereplying with the added proximity message. This message received by the1,090 MHz receiver-decoder on the aircraft which is not then in theradar beam, will trigger the transponder on that aircraft, causing it totransmit a reply, not solicited by a direct SSR interrogation, but bythe other aircraft's proximity message. The other aircraft, that is theone presently in the radar beam, will receive the special transmissionat a time following its own SSR-solicited transmission by an intervalcorresponding to the direct slant range Y between the two aircraft. Thuseither aircraft, if equipped with a range computer 70, is .[.provid.]..Iadd.provided .Iaddend.with slant range information updated with eachrotation of the radar beam.

The range computer also provides differential SSR range X. Note thatboth Y and X are available on the equipped aircraft even if the otheraircraft does not carry a range computer. Similarly, an aircraft with anown SSR bearing computer 76 and an other's bearing computer 75 willobtain φ and B angle information from another that carries only thetransponder 60, receiver-decoder 63 and detector system 66.

Referring to FIG. 3, the 1090 MHZ receiver-decoder 63 of FIG. 1 includesa 1090 MHz receiver 10, a reply frame decoder 11 and an altitude decoder43, all of which may be the same as the respective correspondinglydesignated elements of the system described in said copending U.S. Pat.application Ser. No. 130,952, now U.S. Pat. No. 3,735,408. In addition,a proximity decoder 81 is provided for producing an output pulsewhenever a proximity coded reply signal is received by receiver 10.

The widened common azimuth sector proximity detector includes resettablegate signal generators 3, 12, 35 and 49, AND 45, all the same as therespective corresponding designated elements of the system described inU.S. Pat. No. 3,735,408, and interconnected in the same way. The outputof AND gate 52 is applied to the start input terminal of a resettablegate signal generator 82 designed for a gate time interval of severalSSR beam rotation periods, say 12 seconds. The output of a gategenerator 82 supplies the proximity encode input 61 to the transponder60, and energizes the proximity indicator 53.

The common azimuth sector and range warning signal on line 42 at theoutput of AND gate 39 goes to an AND gate 83, which receives the outputof proximity decoder 81 on line 65 as another input. Simultaneouspresence of both inputs to AND gate 83 produces an output pulse for theexternal trigger input 62 of the transponder 60.

An OR gate 84 is connected to supply the F1 pulse on line 67 or thedecoded P2 pulse on line 68 if either is present, or both if both arepresent, to the gate generator 35 by way of OR gate 34, and to the phaselocked PRF generator 69 by way of a PRF selector 85. The PRF selectorconsists of an AND gate 86 and a delay device 87 connected as shown inFIG. 4. The delay 87 is made equal to the pulse period frequency, thatis, to the interval between successive interrogations of a selected SSR.

Each SSR is assigned a characteristic PRF to distinguish itstransmissions from those of others. The delay 87 may be adjusted by theaircraft operator to select the transmissions of a favorably locatedSSR. Each delay pulse reaches the AND circuit coincidentally with thenext undelayed pulse, producing an output pulse.

Referring to FIG. 5, the phase locked PRF generator 69 of FIG. 1includes an oscillator 88 with frequency control means 89 that can beadjusted, for example by selection of an appropriate crystal, to thedesired PRF. A voltage controlled reactance device 90, for example avaractor diode, is coupled to the frequency control 89 to control thephase of the oscillator 88 in known manner. The oscillator output iscoupled to a phase detector 91, which also receives the selected F1 ordecoded P2 pulses from the PRF selector.

When the aircraft is within the SLS coverage area of the selected radar,decoded P2 pulses are present continuously. Any phase difference betweenthese and the output of oscillator 88 is detected by the phase detector91, which automatically adjusts the voltage controlled reactance deviceto null the difference. The oscillator 88 drives a pulse generally 92 toproduce a continuous train of pulses, hereinafter referred to as"synthetic F1" pulses, that are phase locked to the selected radar PRF.

When the aircraft is outside the SLS coverage, a burst of about twentyactual F1 pulses occurs during the dwell time of the main beam. Theseadjust the phase of the oscillator 88 once during each beam rotation.The reactance device 90 is designed in known manner to hold itsadjustment between bursts. An AND gate 94, controlled by a resettablegate signal generator 93, couples the oscillator 88 to the pulsegenerator 92. The gate generator 93 is designed for a gate time intervalsomewhat longer than one SSR beam rotation, say four seconds. When no F1or decoded P2 pulses occur within about four seconds after the mostrecent burst, the AND gate 94 is disabled, disconnecting the oscillator88 from the pulse generator 92 and stopping it.

Returning to FIG. 3, the synthetic F1 output of the phase locked PRFgenerator 69, when present, supplies an alternative input by way of ORgate 34 to the range warning gate generator 35. The synthetic F1 alsogoes through AND gate 120 to the start input terminal of an intervaltimer 95, which is one of the elements of the range computer 70 ofFIG. 1. A second input to gate 120 is taken from the common azimuthsector range warning line 42. The stop input terminal of interval.[.time.]. .Iadd.timer .Iaddend.95 receives the output of an AND gate96, which has one input from the common azimuth and range warning line42 and another input from the reply frame decoder 11, through a PRFselector 97. This selector, like the selector 85, is adjusted to passonly the repetition frequency of a desired radar.

The range computer also includes another interval timer 98, whichreceives its start input from an AND gate 99 connected to the F1 line 67and the proximity signal gate 82, and its stop input from the proximitydecode 81. Gate 99 receives a third input from a 5-millisecondresettable gate generator 109, connected to be started by a proximitydecode pulse on line 65. Outputs of the interval timers 95 and 98 areapplied to a subtractor 100, and the output of interval .[.time.]..Iadd.timer .Iaddend.98 is displayed on indicator 74.

Referring now to FIG. 6, the interval timers 95 and 98 may be digitaldevices each including a counter 101, a buffer 102, AND gates 103 and104, a control flip-flop 105, and delay .[.device.]. .Iadd.devices.Iaddend.106 amd 107. A common or system clock pulse generator 108provides one input to AND gate 103.

A pulse applied to the start input terminal sets the flip-flop 105,energizing its 1 output terminal and enabling gate 103 to conduct clockpulses to the counter 101. The counter continues to count until a pulseis applied to the stop input terminal, clearing the flip-flop anddeenergizing its 1 output to disable the AND gate 103 and stop thecounter. The accumulated count at this time represents the length of thetime interval between the start and stop input pulses.

After a brief delay in device 106, the stop pulse enables gate 104 totransfer the accumulated count into buffer 102. Gate 104 may be amultiple gate arranged in known manner to effect parallel transfer, ormay be a known arrangement for slower, but adequately rapid serialtransfer. In either case, the buffer 102 is simply forced into a staterepresenting the count most recently transferred to it, holding thatstate until forced into another that represents a new, updated count.

Following a further delay in device 107, long enough to complete thetransfer, the stop input pulse clears the counter 101. The output of thebuffer, which may be either in digital or analog form, represents themost recently measured interval continuously until again updated.

Returning to FIG. 3, the interval timer 98 operates only when aproximity condition has been detected, producing an output from gategenerator 82, and proximity messages are being received from anotheraircraft, producing an output from gate generator 109. These two gatesignals enable the AND gate 99 to pass F1 pulses to start the timer 98.Each next following pulse from the proximity decoder 81 stops the timer,which thus measures the .[.inerval.]. .Iadd.interval .Iaddend.betweenthe two pulses. This interval, taking system delays into account, is theround trip radio transit time between the two aircraft, and is thereforea measure of the direct slant range Y.

It should be noted that the described range measuring operation canoccur between two suitably equipped aircraft in response to any SSR thatilluminates them sequentially. Two or more such radars can cause suchranging without interference, except at the extremely unusual times whenboth beams point simultaneously into the proximity space. Thatsituation, when it does occur, can persist only temporarily because eachradar has a different assigned beam .[.roation.]. rotation period andpulse repetition period.

The interval timer 95 operates only when a common azimuth sector rangewarning signal exists on line 42, enabling AND gates 96 and 120, the PRFgenerator 69 is locked to a selected radar, producing synthetic F1pulses, and reply frames are being received from another aircraftinterrogated by the same selected radar. Under these conditions, theinterval timer is started by each synthetic F1 pulse and stopped by eachdecoded reply frame pulse that passes the PRF selector 97.

The measured interval is that between one's own decoded interrogation orthe synthetic F1 and the reception of the other's reply to thecorresponding interrogation. This interval, taking system delays intoaccount, is a measure of Y+ X, the algebraic sum of the slant range andthe differential SSR range. The output of interval timer 95 goes to thesubtractor 100, where the difference between it and that of intervaltimer 98 produces a representation of the differential SSR range X, tobe utilized by the other's bearing computer.

Turning to the upper portion of FIG. 3, the own bearing computer 76 ofFIG. 1 comprises PRF selectors 121 and 122, envelope detectors 123 and124, delay device 125, interval timers 126 and 127, divider 128 andfunction generator 129. The interval times may be like those describedabove, but designed for operation on a large time scale, measuringintervals of up to a radar beam rotation period, say four seconds.Alternatively, they may be simple electromechanical clock devices ofknown type. The envelope detectors are diode rectifiers with low passfilters, or any other convenient means for converting pulse bursts intosingle, preferably longer pulses.

In operation, each North pulse from the selected SSR first stopsinterval timer 126 if it has been running, then after a brief delay indevice 125, starts both timers 126 and 127. The next subsequent burst ofF1 pulses, occurring as the radar beam sweeps by the aircraft, stopstimer 127, which remains stopped until the next North pulse occurs.

The output of timer 126, designated N, represents the length of timerequired for the radar beam to make a complete revolution. The output oftimer 127, designated M, represents the length of time required for thebeam to rotate from magnetic North to the line of position of theaircraft from the radar. These outputs are applied to the divider 128,which in turn produces an output representing the quotient M/N.

The quantity M/N has a value between zero and unity representing themagnetic bearing φ of the aircraft from the SSR as a fraction of acomplete circle, i.e., 360°. The representation may be digital oranalog, electrical or mechanical, depending upon the specific design ofthe timers 126 and 127 and the divider 128. The function generator 129converts this representation to a form suitable for display by indicator77 and for utilization in the other's bearing computer. It is noted thatthe computed value of φ is independent of the individual beam rotationrate of the selected SSR.

The other's bearing computer 75 of FIG. 1, appearing generally in thelower right hand portion of FIG. 3, includes a divider 130, a functiongenerator 131, an algebraic adding device 132, an algebraic substractingdevice 133, and a lead-lag logic device 134. The usual magnetic compass135 provides own heading, H, information for the computation.

Referring to FIG. 2, it is seen that the angle θ between own SSR line ofposition 80 and the line from own craft to other craft, measuredclockwise from the extension of line 80 past own location 72, is lessthan 90°. When the other aircraft is closer to the SSR, θ as thusmeasured is between 90° and 270°. The differential SSR range X isconsidered positive when the other craft is farther from the SSR, andnegative when the other is nearer.

This sign convention is automatically taken into account by the normaloperation of the interval timer 95 and subtractor 100 of FIG. 3, becauseof the differential transit time measured by the timer 95 isproportional to Y+ |X| when the other craft is farther, and to Y- |X|when the other is nearer. Thus, when Y is subtracted from Y± |X| is thesubtractor 100, the difference X is of the appropriate sign.

Again referring to FIG. 2, all SSR beams rotate clockwise as viewed fromabove, as indicated by the arrow 136. When the aircraft is illuminatedbefore the other as would occur with the .Iadd.own .Iaddend.positionsshown, the angle θ is between zero and 180°. When the other craft isilluminated first, θ lies between 180° and 360°. The first mentionedcondition, shown, is called "lead." The other, not shown, is called"lag." Adopting the convention that Y is positive under the leadcondition and negative under the lag condition, the sign of Y isdetermined by the lead-lag logic device 134.

Referring to FIG. 7, the lead-lag logic comprises AND gates 137 and 138,and flip-flops 139 and 140. A North pulse signal taken from the outputof the envelope detector 123 clears both flip-flops, energizing their 0outputs and enabling both AND gates. Gate 137 is connected to receivedetected F1 burst signals from envelope detector 124, and gate 138 isconnected to receive decoded reply frame pulses in the output of ANDgate 96 of FIG. 3.

After a North pulse signal occurs while the beam of the selected radaris pointing North, an F1 signal will appear while the beam points at theown aircraft and a reply frame signal will appear when the beam pointsat the other aircraft. When the F1 signal occurs first, flip-flop 139 isset, energizing its 1 output terminal to indicate a lead condition, anddeenergizing its 0 output terminal. This disables AND gate 138 toprevent a subsequent reply frame signal from setting flip-flop 140.

When a reply frame signal occurs before the F1 signal, flip-flop 140sets, energizing its 1 output to indicate a lag condition, anddeenergizing its 0 output to prevent setting of flip-flop 139 by asubsequent F1 signal. Accordingly, the sign of Y is determined by whichof the flip-flop 1 outputs is energized.

Returning to FIG. 3, the Y sign information from lead-lag logic device134 and the output of divider 130, representing the quotient X/Y withthe X sign, are applied to the function generator 131, which may be adigital or analog device of known type that produces an outputrepresenting the angle cos⁻ 1 X/Y, including its quadrantal position.This angle is a close approximation, .[.withint.]. .Iadd.within.Iaddend.two or three degrees in a typical situation, of the angle θ.

The adding device combines the representations of θ and φ to produce anoutput representing θ + φ, which, as shown in FIG. 2, is the magneticbearing from the own aircraft to the other aircraft. A similarrepresentation of own magnetic heading H, provided by the compass 135,.[.is.]. .Iadd.as .Iaddend.subtracted in the subtractor 133 to providean output representing θ + φ - H, which is seen in FIG. 2, is the othercraft's bearing from own craft's heading, B. This representation,exhibited by display device 78, indicates directly the line of sight toan intruder aircraft with respect to the own craft's longitudinal axis.

I claim:
 1. A method of determining at an own transponder station the bearing angle φ of said own station from a selected .Iadd.one of a plurality of .Iaddend.secondary surveillance .[.radar.]. .Iadd.radars .Iaddend.(SSR) that .Iadd.transmit directional interrogation signals of the same frequency and that .Iaddend.omniazimuthally .[.transmits a.]. .Iadd.transmit .Iaddend.reference .[.signal.]..Iadd.signals .Iaddend.as the main radar .[.beam sweeps.]. .Iadd.beams thereof sweep .Iaddend.through a standardized reference direction, such as magnetic North, said method comprising the steps of:(a) receiving said reference signals .[.,.]. .Iadd.from said radars, .Iaddend. (b) receiving interrogation signals from said .[.radar.]. .Iadd.radars .Iaddend.as the main .[.beam sweeps.]. .Iadd.beams thereof sweep .Iaddend.by the own location, (c) .[.measuring the time interval between successive reception of one of said reference and said interrogation signals and producing a quantitative first representation of the main beam rotation interval,.]. .Iadd.selecting the reference and interrogation signals received from one of said secondary surveillance radars, .Iaddend. (d) .[.measuring and producing a quantitative second representation of the time interval between reception of a reference signal and reception of the next subsequently received interrogation signal,.]. .Iadd.measuring the time interval between successive reception of one of said selected reference and said selected interrogation signals and producing a quantitative first representation of the main beam rotation interval, .Iaddend. (e) .[.dividing the value of said second representation by that of said first representation to produce the quotient of said values, and.]. .Iadd.measuring and producing a quantitative second representation of the time interval between reception of one of said selected reference and said selected interrogation signals and the next subsequently received other one of said signals, .Iaddend. (f) .[.producing a quantitative representation φ of said quotient..]. .Iadd.dividing the value of said second representation by that of said first representation to produce the quotient of said values, and (g) producing a quantitative representation φ of said quotient. .Iaddend.
 2. Apparatus for determining at an own transponder station the bearing angle φ of said own station from a selected .[.azimuthally scanning radar that omnidirectionally transmits a.]. .Iadd.one of a plurality of secondary surveillance radars (SSR) that transmit directional interrogation signals of the same frequency and that omniazimuthally transmit .Iaddend.reference .[.signal when its directional beam points in a standard.]. .Iadd.signals as the main radar beams thereof sweep through a standardized .Iaddend.reference direction, such as .Iadd.magnetic .Iaddend.North, said apparatus comprising:(a) means for receiving said reference .[.signal.]. .Iadd.signals .Iaddend.and for receiving the .[.normal.]. interrogation transmission signals of said .Iadd.radars .Iaddend.as the radar .[.beam sweeps.]. .Iadd.beams sweep .Iaddend.by the own location, (b) .[.a first interval timer for measuring the interval between successive reception of one of said reference and said normal transmission signals and producing a quantitative representation N of said interval,.]. .Iadd.means for selecting the reference and interrogation signals received from one of said secondary surveillance radars, .Iaddend. (c) .[.a second interval timer for measuring and producing quantitative representation M of the interval between reception of a reference signal and a normal transmission signal,.]. .Iadd.a first interval timer for measuring the interval between successive reception of one of said selected reference and said selected interrogation signals and producing a quantitative representation N of said interval, .Iaddend. (d) .[.divider means for computing the quotient M/N, and.]. .Iadd.a second interval timer for measuring and producing a quantitative second representation M of the time interval between reception of one of said selected reference and said selected interrogation signals and the next subsequently received other one of said signals, .Iaddend. (e) .[.means for producing a quantitative representation φ of said quotient..]. .Iadd.divider means for computing the quotient M/N, and (f) means for producing a quantitative representation φ of said quotient. .Iaddend.
 3. A method as in claim 1 for additionally producing at said own transponder station a representation of the angular relationship between the bearing of said own transponder station from said selected SSR and the bearing from said selected SSR of another transponder station within a common azimuthal sector with said own transponder station, including the further steps of:.[.(g).]. .Iadd.(h) .Iaddend.receiving at said own transponder station reply signals transmitted by said other transponder station in response to said interrogation signals as said main beam sweeps by said other transponder station, .[.(h).]. .Iadd.(i) .Iaddend.determining the time relationship between said interrogation signals and said reply signals, and .[.(i).]. .Iadd.(j) .Iaddend.producing, in accordance with said time relationship, a representation of said angular relationship between said bearing angles from said selected SSR.
 4. The method according to claim 3 wherein step .[.(i).]. .Iadd.(j) .Iaddend.comprises the steps of:.[.(j).]. .Iadd.(k) .Iaddend.producing a lead signal in response to the prior occurrence of said interrogation signals immediately subsequent ot said reference signals, and .[.(k).]. .Iadd.(1) .Iaddend.producing a lag signal in response to the prior occurrence of said reply signals immediately subsequent to said reference signals.
 5. Apparatus according to claim 2 for additionally producing at said own transponder station a representation of the angular relationship between the bearing of said own transponder station from said selected SSR and the bearing from said selected SSR of another transponder station within a common azimuthal sector with said own transponder station, further comprising:.[.(f).]. .Iadd.(g) .Iaddend.means for receiving at said own transponder station reply signals transmitted by said other transponder station in response to said interrogation signals as said main beam sweeps by said other transponder station, .[.(g).]. .Iadd.(h) .Iaddend.means for determining the time relationship between said interrogation signals and said reply signals, and .[.(h).]. .Iadd.(i) .Iaddend.means for producing, in accordance with said time relationship, a representation of said angular relationship between said bearing angles from said selected SSR.
 6. Apparatus according to claim 5 wherein the means .[.(h).]. .Iadd.(i) .Iaddend.comprises:.[.(i).]. .Iadd.(j) .Iaddend.means for producing a lead signal in response to the prior occurrence of said interrogation signals immediately subsequent to said reference signals, and .[.(j).]. .Iadd.(k) .Iaddend.means for producing a lag signal in response to the prior occurrence of said reply signals immediately subsequent to said reference signals. .Iadd.
 7. The method according the claim 1 wherein the reference and interrogation signals are selected in accordance with the pulse repetition characteristic associated with said one of said secondary surveillance radars. .Iaddend. .Iadd.
 8. The apparatus according to claim 2, wherein means (b) selects the reference and interrogation signals in accordance with the pulse repetition characteristic associated with said one of said secondary surveillance radars. .Iaddend. 